In April 2010, BP's Deepwater Horizon drilling rig in the Gulf of Mexico exploded and sank. With the pipe that had once channelled oil 1,400 metres up from the sea floor now broken, some 4.9 million barrels of oil, and an equivalent volume of gas, spewed out over three months, according to the US government. BP added around 9 million litres of chemical dispersants to the oil, roughly a third of it at depth1.

The disaster happened near the nutrient-rich outflow from the Mississippi River delta, one of the most productive areas in a prolific gulf. Some 1,728 species feed and reproduce in this area, and many were breeding at the time, exposing vulnerable larvae and young to toxic oil. Nature examines the damage caused and response to the disaster one year on.

Is the clean-up finished?

The US government estimates that 1.24 million barrels of oil was recovered directly from the broken pipe, skimmed from the surface or burned. It estimates that another 1.2 million barrels "evaporated or dissolved"; 1.1 million formed surface slicks and tar balls, sank to the bottom, or washed up on beaches; 630,000 barrels dispersed naturally; and 770,000 barrels were chemically dispersed2 — although it is worth noting that dispersants do not remove oil from the environment but merely breaks it down into small droplets. Teams responding to the blowout removed oil from beaches, but not marshes, where clean-up efforts do more harm than good, destroying vegetation, compacting the soil, and driving oil into sediments, where it degrades more slowly.

Although the federal government's Operational Science and Advisory Team reported in December that there was "no actionable oil in the water or sediments of the deep water or offshore zones", it acknowledged that "quantitative estimates of remaining oil" were beyond its scope3. Various studies indicate that significant quantities of oil remain at a depth of about 1,100 metres, and possibly on the sea floor4. As recently as last week, oil was observed to still be present in marshes in Louisiana's Barataria Bay.

"The clean-up won't be complete and we won't know the full environmental consequences for at least 40 years," says Tierra Curry, a biologist at the Center for Biological Diversity in Flagstaff, Arizona.

How much damage has been done to the environment?

The complex, legally entangled federal Natural Resource Damage Assessment — coordinated by an arm of the National Oceanic and Atmospheric Administration — is ongoing, so the official jury remains out. But, says Curry, "damage is widespread and will persist for decades".But oil and dispersants are toxic to both shallow and deep ecosystems, according to Larry McKinney, executive director of Texas A&M University's Harte Research Institute for Gulf of Mexico Studies, who predicts the spill's effects will last for decades. The government counted significant numbers of dead animals: 6,104 birds, 609 sea turtles, and 100 marine mammals5. But that only includes animals collected, Curry stresses. Actual mortality is likely to be much higher: scientists estimate that the carcasses gathered so far represent a fifth of the actual mortality figure for turtles, and at most 6% of cetaceans6. The majority of animals that die either sink or are eaten, scientists explain, with only a tiny percentage washing ashore or being spotted at sea by observers.

Long-term effects will be harder to detect, but more insidious. If oil persists in ocean and marsh sediments, plants and animals will be exposed to its effects, and it will inevitably enter the food chain. "Some bird populations haven't recovered more than 20 years after the Exxon Valdez spill in Alaska due to food chain disruption," says Greg Butcher, director of bird conservation for the National Audubon Society in Washington DC.

Studies have shown that dispersed oil is more toxic than oil or dispersant alone, and dispersant chemicals had never before been used at depth, so the effects are not yet known. Scientists are also still sorting out the effects of oil on the deep-sea environment.

What caused the accident?

Human error and equipment failure. "Complex systems such as deepwater drilling rigs fail in complex ways," says Tad Patzek, chairman of the petroleum and geosystems engineering department at the University of Texas, Austin. "A confluence of many factors caused the well to blow," he explains; these factors include cement failing to set, an insufficient rate of cement injection, misinterpretation of a negative-pressure test, and not setting a cement plug to isolate the bottom of the well before the test. Many point fingers at the failed blowout preventer, but Patzek says that the rig explosion damaged the drill pipe so severely it failed.

He lays ultimate blame on bad management, including lack of communication, changes to work flow and orders, and inadequate training. "In my mind, the biggest overall failure was not working according to a single imperative — safety."

Is drilling in deep water still allowed in the Gulf?

"We will drill in deep water because that is where the oil is, and we need the oil," says Patzek. In southern Louisiana, hit hardest by the spill, oil production represents a major part of the economy. Michel Claudet, president of Terrebonne Parish, says many of his constituents supported the federal government's lifting of the drilling moratorium on 12 October 2010.

On 8 April 2011, the government approved the tenth permit for a post-spill deepwater well. Currently, three bills in the US House of Representatives would require more leases in the Gulf and off the Atlantic and West coasts and a boost in offshore production.

What measures have been put in place to stop it from happening again?

The Minerals Management Service, which regulated offshore drilling, has since gained a new director, Michael Bromwich, and name, Bureau of Ocean Energy Management, Regulation and Enforcement (BOEMRE). But environmental groups claim the changes are purely cosmetic, and Bromwich told The New York Times that his agency lacks adequate funding, personnel and enforcement tools.

A commission named by US President Barack Obama to investigate the accident urged more funding for scientific research and more scientific assessments of offshore drilling applications. Patzek serves on BOEMRE's Ocean Energy Safety Advisory Committee, which makes recommendations on offshore energy safety and facilitates collaborative research and training.

The accident made it clear that emergency equipment needs to be designed, constructed and staged at ports, ready on a moment's notice, he adds. That has been done. "We have used the experience from this disaster and now have a lot more hardware. We are more prepared. Much of this technology existed but was not put together and not tested. BP tested it under fire and came up with the best design that capped the well."